The present invention relates to a suction device for securely holding plates by sub-atmospheric pressure in a processing position, and for transporting the plates through a plurality of processing stations.
Suction devices of this type are employed in units for processing printing plates, wherein an individual printing plate is transported from a plate magazine to an exposure stage. A suction device of this type is an especially important component.
In the production of printing plates, the plates are initially charged and then exposed, imagewise, with an original which is to be copied. For this purpose, it is necessary that the printing plate lies as flat as possible in the exposure station.
An electrophotographic continuous copying machine for recording material in sheet form is described in German Auslegeschrift No. 1,597,858. This machine possesses a suction-grab device for lifting the uppermost sheet from a stack, and for feeding the sheet to a transport device. The latter comprises a transport carriage, on the underside of which a suction device, operated at sub-atmospheric pressure, is provided for holding the recording material.
In this known continuously-running machine, time is wasted in view of the fact that the recording material is lifted from the stack by a suction device which is separate from the transport device. This suction device hands over the recording material to the transport device, operated with the use of subatmospheric pressure, of the transport carriage, and the latter then further transports the recording material to the charging device, and to the processing stations which are sequentially arranged behind the charging device.
U.S. Pat. No. 3,330,555 discloses an electrophotographic copying machine, in which the copy sheets are guided to a vacuum-type suction plate.
In the printing-plate processing appliance described in German patent No. 2,462,216, the uppermost printing plate is removed from a plate magazine by a transport carriage which possesses a vacuum plate on the underside. The transport carriage is driven toward the exposure stage, and as soon as it has reached a position above the exposure stage, it is lowered onto the stage, and the sub-atmospheric pressure is removed from the vacuum plate, so that the printing plate is released and is deposited onto the exposure stage. The exposure stage is likewise equipped with a vacuum plate, to which sub-atmospheric pressure is applied as soon as the printing plate rests on it. The printing plate is thereby pressed securely against the exposure stage for the exposure operation. With this arrangement, it is found that the positioning of the pre-sensitized printing plates is insufficiently flat. This is brought about by the fact that these plates are generally produced from aluminum strip material which is wound on cores, whereby elastic deformations are caused in the structure of the material, which give rise to the tendency of the printing plates to roll up. As a result, the printing plate is incompletely held by suction against the vacuum plate of the exposure station, or of the developing station. This factor can give rise to lack of sharpness during the exposure and/or developing process.
In the known machines, time is wasted, above all, by the fact that a certain time is required for the printing plate to be released by the vacuum plate of the processing station in question, since the subatmospheric pressure in the vacuum plate cannot be released suddenly, but requires a definite time interval.
Additionally in the known machine, the device for transporting the printing plate must travel back to the plate magazine from the exposure station, in order to pick up another printing plate from the magazine and transport it into the exposure station. Since, in the known machines the individual stations for exposing, developing, fixing, decoating and, if appropriate, die-cutting the printing plate are constructed one behind another, the space requirement is large. In addition, transport devices, which operate, for example, by means of compressed air or electric motors, and which may be different in the same machine, are provided for the transport of the printing plates from one processing station to the next processing station. Transporting the printing plates between the individual processing stations increases the processing time of the printing plates and, in addition, the machine becomes increasingly susceptible to breakdowns, due to the employment of different transport-device systems, instead of a single transport system capable of conveying the printng plate to the various processing stations.